Elimination of N-Glycolylneuraminic Acid From Animal Products For Human Use

ABSTRACT

The application is in the field of transgenic (non-human) organisms, sialic acid chemistry, metabolism and antigenicity. More particularly, the invention is related to a method to produce Neu5Gc-free animals and products therefrom comprising disrupting the CMAH gene and thereby reducing or eliminating Neu5Gc from biological material of non-humans.

GOVERNMENT INTEREST

This invention was made with government support from the NationalInstitutes of Health under grant number R01CA38701.

TECHNICAL FIELD

The application is in the field of transgenic (non-human) organisms,sialic acid chemistry, metabolism and antigenicity.

BACKGROUND

All cells are covered with a dense and complex array of sugar chains.Sialic acids (Sias) are a family of nine-carbon sugars that aretypically present at the outermost units of these sugar chains. Byvirtue of their terminal position, sialic acids act as binding sites formany exogenous and endogenous receptors such as the Influenza virusesand the Siglec family of endogenous proteins. Such sugars are thususeful drug targets for the prevention and treatment of infection. Theyare also involved in various biological and pathological processes suchas neuronal plasticity and cancer metastasis. In many of theseinstances, the precise structures of the sialic acid and the residues itis attached to play critical roles. Thus, studying sialic acid functionsis of great biological importance. In addition, many sialic acids areobtained through certain dietary sources (red meat and diary products),and may also be associated with certain disease states, such as cancerand heart disease.

SUMMARY OF THE INVENTION

The application is in the field of transgenic (non-human) organisms,sialic acid chemistry, metabolism and antigenicity.

The animals contemplated for use in the practice of the invention arethose animals generally regarded as useful for the processing of foodstuffs, i.e. avian such as meat bred and egg laying chicken and turkey,ovine such as lamb, bovine such as beef cattle and milk cows, piscineand porcine.

In one embodiment, the invention is a method to produce Neu5Gc-freeanimals and products therefrom comprising disrupting the CMAH gene andthereby reducing or eliminating Neu5Gc from biological material ofnon-humans. The CMAH gene may be disrupted by frame-shift mutation orthe cre-lox system. The biological material can be virtually anynon-human organic material suspected of containing Neu5Gc, such as afood sample (for example, red meat or a dairy product) or a clinicalsample. The clinical sample may be from any non-human animal source,such as avian, ovine, bovine, piscine and porcine. Non-human clinicalsamples can be from any body fluid or tissue, such as serum, muscletissue and milk, etc. The Neu5Gc-free animals may be for consumption byhumans, for the purpose of culturing human cells in laboratories and inthe biotechnology industry, and for products for use in the cosmeticindustry.

In another embodiment, the invention is a transgenic non-human animalcomprising Neu5Gc-free serum, muscle tissue and milk. The transgenicanimal may be avian, ovine, bovine, piscine and porcine. Additionally,the transgenic animal may be for consumption by humans. The serum may beused to culture human cells in laboratories and in the biotechnologyindustry, and may be used for products in cosmetics which would reducethe risk of immune responses against such products.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the incorporation of free Neu5Gc in human epithelialcells. Caco-2 cells were fed or not fed for 3 days, with Neu5Gc, ManNGcor Neu5Ac, each at 3 mM final concentration. The cells were harvestedand fractionated, and the Sia content of the different fractions wasanalysed by DMB derivatization and LC-MS analysis as described in“Experimental Procedures”. (A) DMB-HPLC profiles of Sia released frommembrane fractions. Peaks indicated by an asterisk (*) respectivelycorrespond to Neu5Gc7Ac, Neu5,7Ac₂, Neu5,8,9Ac₃, Neu5Gc7,9Ac₂,Neu5,7,9Ac₃. MS and MS/MS data for the peaks corresponding to DMB-Neu5Gcand DMB-Neu5Ac from the membrane bound Sia of ManNGc and Neu5Gc-fedCaco-2 cells were obtained (not shown, see text for discussion) (B)Proportion of Neu5Gc (expressed as percent of total Sia) in thedifferent fractions of Caco-2 cells. Quantitative information (pmol Siaper mg protein) can be found in a “Supplemental Data” file. TH: Totalhomogenate; HMW: High Molecular Weight fraction, LMW: Cytosolic LMWfraction.

FIG. 2 depicts free Neu5Gc taken up and incorporated into other types ofhuman cells. Human fibroblast and neuroblastoma cells were fed or not,for 3 days with 3 mM Neu5Gc or Neu5Ac, and the cells then harvested andfractionated as described in “Experimental Procedures”. The Sia contentin the different fractions of the cells were analysed by DMBderivatization followed by HPLC. The proportion of Neu5Gc (expressed aspercent of total Sia) of the different fractions from (A) humanfibroblasts or (B) human neuroblastomas is shown. TH: Total homogenate;HMW: High Molecular Weight fraction, LMW: Cytosolic LMW fraction.

FIG. 3 depicts uptake of free Neu5Gc not specific for human cells. Humanand Chimpanzee EBV-transformed lymphoblasts were fed or not, for 3 dayswith 3 mM Neu5Gc or Neu5Ac, and the cells were then harvested,fractionated and the Sia content in the different fractions analysed byDMB derivatization followed by HPLC. The proportion of Neu5Gc (expressedas percent of total Sia) of the different fractions from (A) humanlymphoblasts and (B) Chimpanzee lymphoblasts is shown. TH: Totalhomogenate; HMW: High Molecular Weight fraction, LMW: Cytosolic LMWfraction.

FIG. 4 depicts free Neu5Ac can compete with the incorporation of freeNeu5Gc. Caco-2 cells grown in human serum, were fed for 3 days with 3 mMNeu5Gc with or without addition in the media of Neu5Ac at 3 mM or 15 mMfinal concentration. The cells were then harvested, fractionated and theSia content in the different fractions analysed by DMB derivatizationfollowed by HPLC. The proportion of Neu5Gc (expressed as percent oftotal Sia) of (A) total homogenate fraction and (B) membrane fraction isshown.

FIG. 5 depicts free Neu5Gc entering cells via endocytic processes.Caco-2 cells grown in human serum were fed for 3 days with 3 mM Neu5Gcin the presence or absence of various inhibitors of endocytic pathways.The cells were then harvested, fractionated and the Sia content in thedifferent fractions analysed by DMB derivatization followed by HPLC. Theproportion of Neu5Gc (as percent of total Sia) of (A) total homogenatefraction and (B) membrane fraction is shown.

FIG. 6 depicts the lysosomal sialic acid transporter is involved in themetabolic incorporation of free Neu5Gc. Wild-type (WT) and lysosomalSialic acid transporter mutant human fibroblasts (GM08496 and GM05520)grown in human serum, were fed for 3 days with 3 mM Neu5Gc, ManNGc orNeu5Ac. The cells were then harvested, fractionated and the Sia contentin the different fractions analysed by DMB derivatization followed byHPLC. (A) The proportion of Neu5Gc (expressed as percent of total Sia)in the membrane fraction is shown. (B) Western blot analysis of proteinsextracted from the membrane fractions of wild-type and GM05520 mutanthuman fibroblasts, using an anti-Neu5Gc antibody, after Neu5Gc or ManNGefeeding.

FIG. 7 depicts the lysosomal sialidase and sialic acid transporter arerequired for metabolic incorporation of Neu5Gc from serumglycoconjugates. WT fibroblasts, lysosomal sialidase mutant fibroblasts(GM01718) and Sia transporter mutant fibroblasts (GM05520) wereincubated with 10% FCS plus 20% horse serum (rich sources ofglycosidically-bound Neu5Gc) for 3 days. (A) Cells were then releasedfrom the flasks, fixed and analyzed for binding of a chicken anti-Neu5Gcantibody, with and without permeabilization by flow cytometry.Neu5Gc-specific MFI is the median fluorescence intensity (MFI) of thechicken anti-Neu5Gc staining with the background secondary alone MFIsubtracted out. At least 5000 cells were counted for each staining. (B)Percent surface Neu5Gc was calculated by dividing the non-permeabilizedNeu5Gc-specific MFI by the permeabilized Neu5Gc-specific MFI, (C) WT,GM01718, and GM05520 fibroblasts were grown on slides for 4 days, fixed,permeabilized and stained with chicken anti-Neu5Gc IgY and mouseanti-LAMP-1 IgG1. Antibody binding was detected with FITC-goatanti-chicken IgY and Cascade Blue-goat anti-mouse IgG.

FIG. 8 depicts proposed pathways for the uptake and incorporation ofNeu5Gc in human cells. The model proposed is based on the data presentedin this study and upon prior literature. The diamond shape represents aNeu5Gc molecule. The thickness of the various arrows suggests therelative importance of various pathways in delivering Neu5Gc into thecell.

FIGS. 9 a-b depict detection of Neu5Gc on Human embryonic stem cells(HESC) Cultured under Conventional Conditions. EGFP transfected HESCwere grown under conventional conditions using a murine feeder layer andmedium containing 20% “KNOCKOUT” serum replacement. a. HESC werereleased with 2 mM EDTA and studied by flow cytometry using a previouslydescribed affinity-purified polyclonal monospecific antibody againstNeu5Gc followed by a secondary Cy5-conjugated anti-chicken IgY antibody.Gray shaded plot: secondary antibody only; thick line: primary andsecondary antibody; thin line: cells incubated with chicken anti-Neu5Gcin the presence of 1% chimp serum that contains Neu5Gc. b. HESC wereisolated by FACS sorting using the intrinsic EGFP fluorescence. Embryoidbodies were derived by removing the feeder layer and growing the HESC inreduced serum medium for 5 days. Both types of cells were fractionatedinto membrane and cytosolic components. Sias were released and analyzedby DMB derivatization and HPLC. A peak corresponding to Neu5Gc is seenin all fractions.

FIGS. 10 a-c depict HESC stably expressing EGFP can remainundifferentiated when NHS is substituted for animal-derived culturemedium components. a. Bright Field Images (BF) of undifferentiated HESC,scale bars correspond to 100 μm. HESC are positive for EGFP and forseveral non-differentiation markers, such as Oct-4, SSEA-3, andTRA-1-60. Staining for nestin, a neural progenitor marker, was negative,indicating the HESC are not differentiated. b. Bright field image ofEB-derived from EGFP-expressing HESC. They maintain the EGFP expressionafter differentiation. c. HESC colonies were positive for alkalinephosphatase (AP) activity, as shown in the fluorescent image, both withthe standard serum replacement (regular) and in human serum. Cells werealso negative for SSEA-1 and for neural markers such as Nestin, Tuj-1,Map2(a+b), NeuN, astrocyte markers GFAP and S100-13; and oligodendrocytemarkers 04, GSM and RIP (data not shown).

FIG. 11 depicts effect of growth in NHS on Neu5Gc content of HESC andEmbryoid Bodies. HESC or embryoid bodies were grown in NHS instead ofthe standard serum replacement. Membrane-bound Sias were studied forpercentage of Neu5Gc as described in the legend to FIG. 1 b. Datarepresent the mean of two different experiments (mean±SD). *: p<0.005,†: p<0.01.

FIG. 12 depicts binding of “Natural” Antibodies from Sera of NormalHuman Donors to HESC. HESC were grown in regular medium or in NHS as inFIG. 3 for 5 days. The cells were released with 2 mM EDTA and thenexposed to a normal serum from a human with a high level of anti-Neu5Gcantibodies (thick line) or from another individual with a low level ofsuch antibodies (thin line) for 55 min, then stained with a secondarygoat anti-human IgG conjugated to Alexa 594, and studied by flowcytometry, with gating on the EGFP-positive HESC. The gray shaded plotshows the result with secondary antibody alone. Ig deposition wasmarkedly reduced when cells were first grown in NHS-containing mediumfor 5 days (although non-specific background levels were increased, seelower panel). The somewhat higher background seen when HESC were grownin NHS-containing medium could be due to a non-specific IgG absorption,but it had no major consequences, such as complement deposition (seeFIG. 5).

FIG. 13 depicts binding of Complement C3b from Human Sera to HESC. HESCwere grown in regular medium or in NHS-containing medium for 5 days andharvested with 2 mM EDTA as in FIG. 3. The cells were then exposed to anormal human serum from an individual with a high level of anti-Neu5Gcantibodies for 15 min at 37° C., and also to the serum from anindividual with a low level of anti-Neu5Gc antibodies. Deposited C3b wasdetected using a goat anti-human C3, then stained with a goat anti-humanC3b conjugated to Alexa 594 and studied by flow cytometry. Control cellsthat were not exposed to the any human sera showed no positive stainingfor C3b (panel a). After exposure to the high level of anti-Neu5Gcantibodies serum, the double fluorescence plot shows that 37% of theEGFP+cells HESC grown in regular medium showed positive staining forhuman C3b (panel c), whereas only 13% of the EGFP+cells grown in humanserum-containing medium were positive for C3b (panel d). In the case ofthe cells that were exposed to the low level of anti-Neu5Gc antibodiesserum, only 22% of the EGFP+cells were positive for human C3b (panel b,note that the levels were much lower since the Y axis is on a logscale).

DETAILED DESCRIPTION

The application is in the field of transgenic (non-human) organisms,sialic acid chemistry, metabolism and antigenicity.

Sialic acids are nine-carbon sugars found on the surface of mostmammalian cells. The two most common forms of sialic acids areN-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc),which differ by only one oxygen atom. Unlike all other mammals, humansare unable to synthesize Neu5Gc due to a mutation in the CMP-Neu5Achydroxylase (CMAH) gene, which converts Neu5Ac to Neu5Gc. Despite thelack of CMAH in genes, small amounts of Neu5Gc have been found inhealthy human tissues as well as malignant tissues. Research has shownthat the incorporated Neu5Gc may originate from dietary sources(Tangvoranuntakul et al., 2003). Red meat like beef, pork and lamb areparticularly rich in Neu5Gc and should be considered the primary sourceof Neu5Gc in human diet. Also dairy products contain Neu5Gc although atsomewhat lower levels than in red meat. Considering that most humansalso have antibodies to Neu5Gc, incorporation of Neu5Gc is hypothesizedto be one of the factors contributing to the health risks associatedwith high consumption of red meat (heart disease, certain types ofcancers). The presence of Neu5Gc in animal products is also a potentialsource of allergenicity. Additionally, when human cells are cultured inserum from animals they can take up and incorporate Neu5Gc, potentiallyresulting in immunological rejection if such cells are used for therapy(e.g. transplantation of human embryonic stem cell-derived grafts). Ithas now been demonstrated that targeted disruption of the CMAH gene inmice completely abolished the expression of Neu5Gc in all tissues aswell as in their secretions. Similar disruption of the CMAH gene indomesticated livestock (cows, pigs, goats etc.) may provide a source ofNeu5Gc-free meat, milk, and other food products, items used in research,e.g. serum, as well as in cosmetics.

By targeting one single gene, CMAH, Neu5Gc can be eliminated from allmouse tissues and secretions, including serum, muscle tissue and milk.Since Neu5Gc is immunogenic to humans, eliminating CMAH fromdomesticated animals would provide a source of Neu5Gc products for humanuse and consumption. For example, a CMAH null cows would avoid uptakeand incorporation of Neu5Gc from ingested meat, milk etc. This is aunique intervention, which would provide humans with a safer source ofbovine food items and other products, and hence reducing the risk ofdeveloping potentially autoreactive antibodies. The absence of Neu5Gc inbovine serum products used for human cell tissue culture would alsoprovide more human-like growth conditions.

The same methodology as was used to knock out CMAH in mice, would beemployed to disrupt CMAH gene expression in domesticated animals. Formice, a frame-shift mutation, similar to the one found in the humanCMAH, was introduced using the Cre-Lox recombination system. Whilenormal wild-type mice express equal levels of Neu5Gc and Neu5Ac in theirmuscle tissue, and approximately 5% Neu5Gc in their milk, we found noevidence for Neu5Gc expression in tissues or milk of the CMAH null mice.Since the mice are otherwise viable and fertile (as are humans), we canpredict that other CMAH nulls animals will also be the same.

The use of Neu5Gc-free products could be applicable to severalcommercial settings. First, if consumption of Neu5Gc indeed turns out tobe a significant risk to human health, meat from Neu5Gc-free animalswould provide a safer alternative source of red meat. Secondly, thiscould replace normal animal serum which are currently used to culturehuman cells in laboratories, and in the biotechnology industry. Thirdly,the use of Neu5Gc-free bovine products in cosmetics would reduce therisk of immune responses against such products.

N-glycolylneuraminic acid (Neu5Gc) is a widely expressed sialic acid inmammalian cells. While humans are genetically deficient in producingNeu5Gc, small amounts are present in human cells in vivo. A dietaryorigin was suggested by human volunteer studies, and by observing thatfree Neu5Gc is metabolically incorporated into cultured human carcinomacells, by unknown mechanisms. We now show that free Neu5Gc uptake alsooccurs in other human and mammalian cells. Inhibitors of certainnon-clathrin mediated endocytic pathways reduce Neu5Gc accumulation.Studies with human mutant cells show that the lysosomal sialic acidtransporter is required for metabolic incorporation of free Neu5Gc.Incorporation of glycosidically-bound Neu5Gc from exogenousglycoconjugates (relevant to human gut epithelial exposure to dietaryNeu5Gc) requires the transporter, as well as the lysosomal sialidase,which presumably acts to release free Neu5Gc. Thus, exogenous Neu5Gcreaches lysosomes via pinocytic/endocytic pathways, and is exported infree form into the cytosol, becoming available for activation andtransfer to glycoconjugates. In contrast, N-glycolylmannosamine (ManNGc)apparently traverses the plasma membrane by passive diffusion andbecomes available for conversion to Neu5Gc in the cytosol. Thismechanism can also explain the metabolic incorporation of chemicallysynthesized unnatural sialic acids, as reported by others. Finally, itis believed that this is the first example of delivery to the cytosol ofan extracellular small molecule that cannot cross the plasmamembrane—utilizing fluid pinocytosis and a specific lysosomaltransporter. The approach could thus potentially be generalized to anysmall molecule that has a specific lysosomal transporter, but not aplasma membrane transporter.

Sialic acid (Sia) is a generic name for a family of acidic nine carbonsugars typically found as the outermost units of glycan chains on thevertebrate cellular glycocalyx and on secreted glycoproteins. Theirlocation and widespread occurrence on all vertebrate cells allow them tobe involved in processes such as pathogen binding, inflammation, immuneresponse and tumor metastasis.

There are more than 50 kinds of Sias known in nature. Most are derivedvia biosynthetic modification of a Sia called N-acetylneuraminic acid(Neu5Ac). The addition of a single oxygen atom to the N-acetyl group ofNeu5Ac gives a very common variation called N-glycolylneuraminic acid(Neu5Gc). The surfaces of most primate cell types studied to date aredominated by two major Sias, which are Neu5Ac and Neu5Gc.

Sialic acids (Sias) are a family of acidic sugars displayed on thesurfaces of all cell types, and on many secreted proteins. The two mostcommon mammalian Sias are N-glycolylneuraminic acid (Neu5Gc) andN-acetylneuraminic acid (Neu5Ac), with Neu5Ac being the metabolicprecursor of Neu5Gc. Humans are genetically unable to produce Neu5Gcfrom Neu5Ac, due to a mutation that occurred after our common ancestorwith great apes. Thus, while human cells have no overall loss of Sias,they express primarily Neu5Ac. However, they can potentially take Neu5Gcup from media containing animal products, activate it into CMP-Neu5Gc,and metabolically incorporate it using the same Golgi transporter andsialyltransferases as CMP-Neu5Ac. Most normal healthy humans havecirculating anti-Neu5Gc antibodies. Thus, xenogenic culture methodologycould compromise transplantation success, due to uptake and expressionof Neu5Gc on the surface of any tissue developed from HESC. Suchincorporation could induce an immune response upon transplantation.

In order for a Sia molecule to get attached to glycoconjugates, it mustfirst be activated by conversion to the sugar nucleotide derivativecytidine-monophosphate-Sia (CMP-Sia). Thus, Sias are converted toCMP-Sias in the nucleus, which then return to the cytosol in order to betransported into the Golgi apparatus, where they serve as high-energydonors for attaching Sias to newly synthesized glycoconjugates on theirway to the cell surface. The biosynthetic transformation of Neu5Ac toNeu5Gc occurs at this sugar nucleotide level, wherein the CMP-Neu5Achydroxylase (CMAH) catalyzes the transfer of an oxygen atom toCMP-Neu5Ac, generating CMP-Neu5Gc. CMP-Neu5Gc can then be transportedinto the Golgi apparatus and used, in the same manner as CMP-Neu5Ac, toadd Neu5Gc to newly synthesized glycoconjugates. Indeed, these twonucleotide sugars appear to be used interchangeably by the Golgi CMP-Siatransporter and by the mammalian sialyltransferases, which transfer Siaresidues to cell surface and secreted glycoconjugates. Neu5Ac or Neu5Gcmolecules that are released from glycoconjugates during lysosomaldegradation processes can also be exported back into the cytosoliccompartment by a specific transporter. There, they are both available assubstrates for conversion to their respective CMP-Sia forms. Again,there appears to be no major difference in their conversion by CMP-SiaSynthases. In this manner, Neu5Gc can be “recycled” for repeated use inGolgi sialylation reactions.

Although Neu5Gc is a major Sia in most mammalian cells, it was longthought to be absent from healthy human tissues. Indeed, humans aregenetically unable to synthesize Neu5Gc, due to an exon deletion/frameshift mutation in the human CMAH gene. It has been estimated that thismutation occurred in the hominid lineage ˜2.5 to 3 million years ago.One dramatic consequence of this human-specific genetic defect appearsto have been the sudden unmasking of the CD33-related Siglecs duringhuman evolution, since the ancestral condition of these molecules was torecognize Neu5Gc.

Despite the absence of any known alternative pathway for the synthesisof Neu5Gc in humans, various groups have used antibodies to claim theexpression of Neu5Gc in human tumors, particularly in variouscarcinomas. Recent studies from our laboratory re-explored this issue,confirming prior reports of Neu5Gc expression in human cancers andextending the finding to normal human tissues, detecting small amountsof Neu5Gc in epithelial and endothelial cells of normal humans.Definitive confirmation resulted from releasing and purifying sialicacids from such tissues, and identifying a fluorescent derivatized formof Neu5Gc by HPLC and mass spectrometry analysis. Moreover, it was shownthat exogenously added free Neu5Gc could be incorporated into culturedhuman carcinoma cells in vitro. In addition, oral ingestion studies ofNeu5Gc in human volunteers were carried out, providing evidence that theNeu5Gc found in human tissues could be originating from dietary sources,particularly red meat and milk products. It now becomes critical tounderstand the pathway(s) for uptake and metabolic incorporation ofNeu5Gc and its potential precursor, N-glycolylmannosamine (ManNGc) intohuman cells.

HESC can potentially generate every body cell type, making themexcellent candidates for cell and tissue replacement therapies. HESC aretypically cultured with animal-derived “serum replacements” on murinefeeder layers. Both these are sources of the non-human sialic acidNeu5Gc, against which many humans have circulating antibodies. Both HESCand derived embryoid bodies metabolically incorporate significantamounts of Neu5Gc under standard conditions. Exposure to human sera withanti-Neu5Gc antibodies resulted in binding of immunoglobulin anddeposition of complement, which would lead to cell killing in vivo.Levels of Neu5Gc on HESC and embryoid bodies dropped after culture inheat-inactivated anti-Neu5Gc-antibody-negative human serum, reducingbinding of antibodies and complement from high titer sera, whileallowing maintenance of the undifferentiated state. Complete eliminationof Neu5Gc would likely require using human serum with human feederlayers, ideally starting with fresh HESC that have never been exposed toanimal products.

The pluripotent abilities of human embryonic stem cells(HESC) havepotential for treating many diseases by transplantation of HESC-derivedtissues. While safety is a major issue regarding infection ortumorigenicity, the possibility of rejection is also of concern. Currentculture methods use animal products, carrying the risk of infection bynon-human pathogens. HESC lines are traditionally cultured onmitotically-inactivated mouse embryonic fibroblasts (so-called “feederlayers”), and in a media containing fetal calf serum. To avoid animalserum, certain proprietary serum replacements are in use. However, thesealso contain animal products. When HESC are removed from the feederlayer and grown in suspension, they differentiate into aggregates calledembryoid bodies (EB). EB are formed by precursors of several celllineages and can be induced to differentiate into many cell types.Although the feeder layer is no longer necessary, EB must still bemaintained in “serum replacement” medium.

The invention provides knockout non-human organisms lacking a functionalCMAH. “Knock-out” refers to partial or complete suppression of theexpression of a protein encoded by an endogenous DNA sequence in a cell.The “knock-out” can be affected by targeted deletion of the whole orpart of a gene encoding a protein in an embryonic stem cell. As aresult, the deletion may prevent or reduce the expression of the proteinin any cell in the whole animal in which it is normally expressed. Forexample, an “CMAH knock-out animal” refers to an animal in which theexpression CMAH has been reduced or suppressed by the introduction of arecombinant nucleic acid molecule that disrupts at least a portion ofthe genomic DNA sequence encoding CMAH.

“Transgenic animal” refers to an animal to which exogenous DNA has beenintroduced while the animal is still in its embryonic stage. In mostcases, the transgenic approach aims at specific modifications of thegenome, e.g., by introducing whole transcriptional units into thegenome, or by up- or down-regulating pre-existing cellular genes. Thetargeted character of certain of these procedures sets transgenictechnologies apart from experimental methods in which random mutationsare conferred to the germline, such as administration of chemicalmutagens or treatment with ionizing solution.

The term “knockout mammal” and the like, refers to a transgenic mammalwherein a given gene has been suppressed by recombination with atargeting vector. It is to be emphasized that the term is intended toinclude all progeny generations. Thus, the founder animal and all F1,F2, F3, and so on, progeny thereof are included.

The term “chimera,” “mosaic,” “chimeric mammal” and the like, refers toa transgenic mammal with a knockout in some of its genome-containingcells.

The term “heterozygote,” “heterozygotic mammal” and the like, refers toa transgenic mammal with a disruption on one of a chromosome pair in allof its genome-containing cells.

The term “homozygote,” “homozygotic mammal” and the like, refers to atransgenic mammal with a disruption on both members of a chromosome pairin all of its genome-containing cells.

A “non-human animal” of the invention includes mammals such as rodents,non-human primates, sheep, dog, amphibians, reptiles, avian such as meatbred and egg laying chicken and turkey, ovine such as lamb, bovine suchas beef cattle and milk cows, piscine and porcine.

Although the invention uses a typical non-human rodent animal (e.g.,including rat and mouse) other animals can similarly be geneticallymodified using the methods and compositions of the invention.

A “mutation” is a detectable change in the genetic material in theanimal, which is transmitted to the animal's progeny. A mutation isusually a change in one or more deoxyribonucleotides, the modificationbeing obtained by, for example, adding, deleting, inverting, orsubstituting nucleotides.

Typically, the genome of the transgenic non-human animal comprises oneor more deletions in one or more exons of the genes and furthercomprises a heterologous selectable marker gene.

In principle, knockout animals may have one or both copies of the genesequence of interest disrupted. In the latter case, in which ahomozygous disruption is present, the mutation is termed a “null”mutation. In the case where only one copy of the nucleic acid sequenceof interest is disrupted, the knockout animal is termed a “heterozygousknockout animal”. The knockout animals of the invention are typicallyhomozygous for the disruption of both CMAH genes being targeted.

It is important to note that it is not necessary to disrupt a gene togenerate a transgenic organism lacking functional expression. Theinvention includes the use of antisense molecules that are transformedinto a cell, such that production of an OAT polypeptide is inhibited.Such an antisense molecule is incorporated into a germ cell as describedmore fully herein operably linked to a promoter such that the antisenseconstruct is expressed in all cells of a transgenic organism.

Techniques for obtaining the transgenic animals of the invention arewell known in the art; the techniques for introducing foreign DNAsequences into the mammalian germ line were originally developed inmice. One route of introducing foreign DNA into a germ line entails thedirect microinjection of linear DNA molecules into a pronucleus of afertilized one-cell egg. Microinjected eggs are subsequently transferredinto the oviducts of pseudopregnant foster mothers and allowed todevelop. About 25% of the progeny mice inherit one or more copies of themicro-injected DNA. Currently, the most frequently used techniques forgenerating chimeric and transgenic animals are based on geneticallyaltered embryonic stem cells or embryonic germ cells. Techniquessuitable for obtaining transgenic animals have been amply described. Asuitable technique for obtaining completely ES cell derived transgenicnon-human animals is described in WO 98/06834.

Knockout animals of the invention can be obtained by standard genetargeting methods as described above, typically by using ES cells. Thus,the invention relates to a method for producing a knockout non-humanmammal comprising (i) providing an embryonic stem (ES) cell from therelevant animal species comprising a first intact CMAH gene; (ii)providing a targeting vector capable of disrupting the intact CMAH gene;(iii) introducing the targeting vector into the ES cells underconditions where the intact CMAH undergoes homologous recombination withthe targeting vector to produce a mutant CMAH gene; (iv) introducing theES cells carrying a disrupted CMAH gene into a blastocyst; (v)implanting the blastocyst into the uterus of pseudopregnant female; (vi)delivering animals from said females, identifying a mutant animal thatcarries the mutant allele and MO selecting for knockout animals andbreeding them.

A “targeting vector” is a vector comprising sequences that can beinserted into the gene to be disrupted, e.g., by homologousrecombination. The targeting vector generally has a 5′ flanking regionand a 3′ flanking region homologous to segments of the gene of interest,surrounding a foreign DNA sequence to be inserted into the gene. Forexample, the foreign DNA sequence may encode a selectable marker, suchas an antibiotics resistance gene. Examples for suitable selectablemarkers are the neomycin resistance gene (NEO) and the hygromycinβ-phosphotransferase gene. The 5′ flanking region and the 3′ flankingregion are homologous to regions within the gene surrounding the portionof the gene to be replaced with the unrelated DNA sequence. DNAcomprising the targeting vector and the native gene of interest arecontacted under conditions that favor homologous recombination. Forexample, the targeting vector and native gene sequence of interest canbe used to transform embryonic stem (ES) cells, in which they cansubsequently undergo homologous recombination.

Thus, a targeting vector refers to a nucleic acid that can be used todecrease or suppress expression of a protein encoded by endogenous DNAsequences in a cell. In a simple example, the knockout construct iscomprised of a CMAH polynucleotide with a deletion in a critical portionof the polynucleotide so that a functional CMAH cannot be expressedtherefrom. Alternatively, a number of termination codons can be added tothe native polynucleotide to cause early termination of the protein oran intron junction can be inactivated. In a typical knockout construct,some portion of the polynucleotide is replaced with a selectable marker(such as the neo gene) so that the polynucleotide can be represented asfollows: CMAH 5′/neo/CMAH 3′, where CMAH 5′ and CMAH 3′, refer togenomic or cDNA sequences which are, respectively, upstream anddownstream relative to a portion of the CMAH polynucleotide and whereneo refers to a neomycin resistance gene.

Proper homologous recombination can be confirmed by Southern blotanalysis of restriction endonuclease digested DNA using, as a probe, anon-disrupted region of the gene. Since the native gene will exhibit arestriction pattern different from that of the disrupted gene, thepresence of a disrupted gene can be determined from the size of therestriction fragments that hybridize to the probe.

In an animal obtained by the methods above, the extent of thecontribution of the ES cells that contain the disrupted CMAH gene to thesomatic tissues of the transgenic animal can be determined visually bychoosing animal strains for the source of the ES cells and blastocystthat have different coat colors.

In a one embodiment, the knockout animals of the invention are mice. Inother embodiments of this invention, the animals are rodents, guineapigs, rabbits, non-human primates, sheep, dog, cow, amphibians,reptiles, avian such as meat bred and egg laying chicken and turkey,ovine such as lamb, bovine such as beef cattle and milk cows, piscineand porcine. The production of knockout animals is described in furtherdetail below.

The invention further provides for transgenic animals, which can be usedfor a variety of purposes, e.g., to identify therapeutics agents forCMAH mediated disorders as well as providing sources of food and foodproducts that lack Neu5Gc.

The transgenic animals can typically contain a transgene, such asreporter gene, under the control of an CMAH promoter or fragmentthereof. Methods for obtaining transgenic and knockout non-human animalsare known in the art. Knock out mice are generated by homologousintegration of a “targeting vector” construct into a mouse embryonicstem cell chromosome which encodes a gene to be knocked out. In oneembodiment, gene targeting, which is a method of using homologousrecombination to modify an animal's genome, can be used to introducechanges into cultured embryonic stem cells. By targeting an CMAH gene ofinterest in ES cells, these changes can be introduced into the germlinesof animals to generate chimeras. The gene targeting procedure isaccomplished by introducing into tissue culture cells a DNA targetingvector that includes a segment homologous to a target CMAH locus, andwhich also includes an intended sequence modification to the CMAHgenomic sequence (e.g., insertion, deletion, point mutation). Thetreated cells are then screened for accurate targeting to identify andisolate those which have been properly targeted.

Generally, the embryonic stem cells (ES cells) used to produce theknockout animals will be of the same species as the knockout animal tobe generated. Thus for example, mouse embryonic stem cells will usuallybe used for generation of knockout mice.

Embryonic stem cells are generated and maintained using methods wellknown to the skilled artisan such as those described by Doetschman etal. (1985) J. Embryol. Exp. Mol. Biol. 87:27-45). Any line of ES cellscan be used, however, the line chosen is typically selected for theability of the cells to integrate into and become part of the germ lineof a developing embryo so as to create germ line transmission of theknockout construct. Thus, any ES cell line that is believed to have thiscapability is suitable for use herein. One mouse strain that istypically used for production of ES cells, is the 129J strain. AnotherES cell line is murine cell line D3 (American Type Culture Collection,catalog no. CKL 1934). Still another ES cell line is the WW6 cell line(Joffe et al. (1995) PNAS 92;7357-7361). The cells are cultured andprepared for knockout construct insertion using methods well known tothe skilled artisan, such as those set forth by Robertson in:Teratocarcinomas and Embryonic Stem Cells: A Practical Approach, E. J.Robertson, ed. IRI. Press, Washington, D.C. [1987]); by Bradley et al.(1986) Current Topics in Devel. Biol. 20:357-371); and by Hogan et al.(Manipulating the Mouse Embryo: A Laboratory Manual, Cold Spring HarborLaboratory Press, Cold Spring Harbor, N.Y. (1986)).

A targeting vector construct refers to a uniquely configured fragment ofnucleic acid which is introduced into a stem cell line and allowed torecombine with the genome at the chromosomal locus of the gene ofinterest to be mutated. Thus a given knock out construct is specific fora given gene to be targeted for disruption. Nonetheless, many commonelements exist among these constructs and these elements are well knownin the art. A typical targeting vector contains nucleic acid fragmentsof not less than about 0.5 kb nor more than about 10.0 kb from both the5′ and the 3′ ends of the genomic locus which encodes the gene to bemutated. These two fragments are separated by an intervening fragment ofnucleic acid which encodes a positive selectable marker, such as theneomycin resistance gene (nee). The resulting nucleic acid fragment,consisting of a nucleic acid from the extreme 5′ end of the genomiclocus linked to a nucleic acid encoding a positive selectable markerwhich is in turn linked to a nucleic acid from the extreme 3′ end of thegenomic locus of interest, omits most of the coding sequence for CMAH orother gene of interest to be knocked out. When the resulting constructrecombines homologously with the chromosome at this locus, it results inthe loss of the omitted coding sequence, otherwise known as thestructural gene, from the genomic locus. A stem cell in which such ahomologous recombination event has taken place can be selected for byvirtue of the stable integration into the genome of the nucleic acid ofthe gene encoding the positive selectable marker and subsequentselection for cells expressing this marker gene in the presence of anappropriate drug (neomycin in this example).

Variations on this basic technique also exist and are well known in theart. For example, a “knock-in” construct refers to the same basicarrangement of a nucleic acid encoding a 5′ genomic locus fragmentlinked to nucleic acid encoding a positive selectable marker which inturn is linked to a nucleic acid encoding a 3′ genomic locus fragment,but which differs in that none of the coding sequence is omitted andthus the 5′ and the 3′ genomic fragments used were initially contiguousbefore being disrupted by the introduction of the nucleic acid encodingthe positive selectable marker gene. This “knock-in” type of constructis thus very useful for the construction of mutant transgenic animalswhen only a limited region of the genomic locus of the gene to bemutated, such as a single exon, is available for cloning and geneticmanipulation. Alternatively, the “knock-in” construct can be used tospecifically eliminate a single functional domain of the targeted gene,resulting in a transgenic animal which expresses a polypeptide of thetargeted gene which is defective in one function, while retaining thefunction of other domains of the encoded polypeptide. This type of“knock-in” mutant frequently has the characteristic of a so-called“dominant negative” mutant because, especially in the case of proteinswhich homomultimerize, it can specifically block the action of (or“poison”) the polypeptide product of the wild-type gene from which itwas derived. In a variation of the knock-in technique, a marker gene isintegrated at the genomic locus of interest such that expression of themarker gene comes under the control of the transcriptional regulatoryelements of the targeted gene. One skilled in the art will be familiarwith useful markers and the means for detecting their presence in agiven cell.

As mentioned above, the homologous recombination of the above described“knock out” and “knock in” constructs is sometimes rare and such aconstruct can insert nonhomologously into a random region of the genomewhere it has no effect on the gene which has been targeted for deletion,and where it can potentially recombine so as to disrupt another genewhich was otherwise not intended to be altered. Such non-homologousrecombination events can be selected against by modifying theabove-mentioned targeting vectors so that they are flanked by negativeselectable markers at either end (particularly through the use of twoallelic variants of the thymidine kinase gene, the polypeptide productof which can be selected against in expressing cell lines in anappropriate tissue culture medium well known in the art—i.e. onecontaining a drug such as 5-bromodeoxyuridine). Non-homologousrecombination between the resulting targeting vector comprising thenegative selectable marker and the genome will usually result in thestable integration of one or both of these negative selectable markergenes and hence cells which have undergone non-homologous recombinationcan be selected against by growth in the appropriate selective media(e.g. media containing a drug such as 5-bromodeoxyuridine for example).Simultaneous selection for the positive selectable marker and againstthe negative selectable marker will result in a vast enrichment forclones in which the knock out construct has recombined homologously atthe locus of the gene intended to be mutated. The presence of thepredicted chromosomal alteration at the targeted gene locus in theresulting knock out stem cell line can be confirmed by means of Southernblot analytical techniques which are well known to those familiar in theart. Alternatively, PCR can be used.

Each targeting vector to be inserted into the cell is linearized.Linearization is accomplished by digesting the DNA with a suitablerestriction endonuclease selected to cut only within the vector sequenceand not the 5′ or 3′ homologous regions or the selectable marker region.

For insertion, the targeting vector is added to the ES cells underappropriate conditions for the insertion method chosen, as is known tothe skilled artisan. For example, if the ES cells are to beelectroporated, the ES cells and targeting vector are exposed to anelectric pulse using an electroporation machine and following themanufacturer's guidelines for use. After electroporation, the ES cellsare typically allowed to recover under suitable incubation conditions.The cells are then screened for the presence of the targeting vector asexplained herein. Where more than one construct is to be introduced intothe ES cell, each targeting vector can be introduced simultaneously orone at a time.

After suitable ES cells containing the knockout construct in the properlocation have been identified by the selection techniques outlinedabove, the cells can be inserted into an embryo. Insertion may beaccomplished in a variety of ways known to the skilled artisan, howeverthe typical method is by microinjection. For microinjection, about 10-30cells are collected into a micropipet and injected into embryos that areat the proper stage of development to permit integration of the foreignES cell containing the recombination construct into the developingembryo. For instance, the transformed ES cells can be microinjected intoblastocytes. The suitable stage of development for the embryo used forinsertion of ES cells is very species dependent, however for mice it isabout 3.5 days. The embryos are obtained by perfusing the uterus ofpregnant females. Suitable methods for accomplishing this are known tothe skilled artisan.

While any embryo of the right stage of development is suitable for use,typical embryos are male. In mice, the typical embryos also have genescoding for a coat color that is different from the coat color encoded bythe ES cell genes. In this way, the offspring can be screened easily forthe presence of the knockout construct by looking for mosaic coat color(indicating that the ES cell was incorporated into the developingembryo). Thus, for example, if the ES cell line carries the genes forwhite fur, the embryo selected will carry genes for black or brown fur.

After the ES cell has been introduced into the embryo, the embryo may beimplanted into the uterus of a pseudopregnant foster mother forgestation. While any foster mother may be used, the foster mother istypically selected for her ability to breed and reproduce well, and forher ability to care for the young. Such foster mothers are typicallyprepared by mating with vasectomized males of the same species. Thestage of the pseudopregnant foster mother is important for successfulimplantation, and it is species dependent. For mice, this stage is about2-3 days pseudopregnant.

Offspring that are born to the foster mother may be screened initiallyfor mosaic coat color where the coat color selection strategy (asdescribed above, and in the appended examples) has been employed. Inaddition, or as an alternative, DNA from tail tissue of the offspringmay be screened for the presence of the knockout construct usingSouthern blots and/or PCR as described above. Offspring that appear tobe mosaics may then be crossed to each other, if they are believed tocarry the knockout construct in their germ line, in order to generatehomozygous knockout animals. Homozygotes may be identified by Southernblotting of equivalent amounts of genomic DNA from mice that are theproduct of this cross, as well as mice that are known heterozygotes andwild type mice.

Other means of identifying and characterizing the knockout offspring areavailable. For example, Northern blots can be used to probe the mRNA forthe presence or absence of transcripts encoding either the gene knockedout, the marker gene, or both. In addition, Western blots can be used toassess the level of expression of the CMAH gene knocked out in varioustissues of the offspring by probing the Western blot with an antibodyagainst the particular CMAH protein, or an antibody against the markergene product (i.e., the presence of Neu5GC using an antibody asidentified in PCT application no. PCT/US2004/022415, incorporated hereinby reference in its entirety). Finally, in situ analysis (such as fixingthe cells and labeling with antibody) and/or FACS (fluorescenceactivated cell sorting) analysis of various cells from the offspring canbe conducted using suitable antibodies to look for the presence orabsence of the knockout construct gene product.

Yet other methods of making knock-out or disruption transgenic animalsare also generally known. See, for example, Manipulating the MouseEmbryo, (Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.,1986). Recombinase dependent knockouts can also be generated, e.g. byhomologous recombination to insert target sequences, such that tissuespecific and/or temporal control of inactivation of an OAT gene can becontrolled by recombinase sequences.

Animals containing more than one knockout construct and/or more than onetransgene expression construct are prepared in any of several ways. Atypical manner of preparation is to generate a series of mammals, eachcontaining one of the desired transgenic phenotypes. Such animals arebred together through a series of crosses, backcrosses and selections,to ultimately generate a single animal containing all desired knockoutconstructs and/or expression constructs, where the animal is otherwisecongenic (genetically identical) to the wild type except for thepresence of the knockout construct(s) and/or transgene(s).

In another aspect, a transgenic animal can be obtained by introducinginto a single stage embryo a targeting vector. The zygote is the besttarget for micro-injection. In the mouse, the male pronucleus reachesthe size of approximately 20 micrometers in diameter which allowsreproducible injection of 1-2 pl of DNA solution. The use of zygotes asa target for gene transfer has an advantage in that in most cases theinjected DNA will be incorporated into the host gene before the firstcleavage (Brinster et al. (1985) PNAS 82:4438-4442). As a consequence,all cells of the transgenic animal will carry the incorporated nucleicacids of the targeting vector. This will in general also be reflected inthe efficient transmission to offspring of the founder since 50% of thegerm cells will harbor the transgene.

Normally, fertilized embryos are incubated in suitable media until thepronuclei appear. At about this time, the nucleotide sequence comprisingthe transgene is introduced into the female or male pronucleus. In somespecies such as mice, the male pronucleus is typically used. Typicallythe exogenous genetic material be added to the male DNA complement ofthe zygote prior to its being processed by the ovum nucleus or thezygote female pronucleus. It is thought that the ovum nucleus or femalepronucleus release molecules which may affect the male DNA complement,perhaps by replacing the protamines of the male DNA with histones,thereby facilitating the combination of the female and male DNAcomplements to form the diploid zygote.

Thus, the exogenous genetic material is typically added to the malecomplement of DNA or any other complement of DNA prior to its beingaffected by the female pronucleus. For example, the exogenous geneticmaterial is added to the early male pronucleus, as soon as possibleafter the formation of the male pronucleus, which is when the male andfemale pronuclei are well separated and both are located close to thecell membrane. Alternatively, the exogenous genetic material could beadded to the nucleus of the sperm after it has been induced to undergodecondensation. Sperm containing the exogenous genetic material can thenbe added to the ovum or the decondensed sperm could be added to the ovumwith the transgene constructs being added as soon as possiblethereafter.

Introduction of the a exogenous nucleic acid (e.g., a targeting vector)into the embryo may be accomplished by any means known in the art suchas, for example, microinjection, electroporation, or lipofection.Following introduction of the exogenous nucleic acid into the embryo,the embryo may be incubated in vitro for varying amounts of time, orreimplanted into the surrogate host, or both. In vitro incubation tomaturity is within the scope of this invention. One common method in toincubate the embryos in vitro for about 1-7 days, depending on thespecies, and then reimplant them into the surrogate host.

For the purposes of this invention a zygote is essentially the formationof a diploid cell which is capable of developing into a completeorganism. Generally, the zygote will be comprised of an egg containing anucleus formed, either naturally or artificially, by the fusion of twohaploid nuclei from a gamete or gametes. Thus, the gamete nuclei must beones which are naturally compatible, i.e., ones which result in a viablezygote capable of undergoing differentiation and developing into afunctioning organism. Generally, a euploid zygote is used. If ananeuploid zygote is obtained, then the number of chromosomes should notvary by more than one with respect to the euploid number of the organismfrom which either gamete originated.

In addition to similar biological considerations, physical ones alsogovern the amount (e.g., volume) of exogenous genetic material which canbe added to the nucleus of the zygote or to the genetic material whichforms a part of the zygote nucleus. If no genetic material is removed,then the amount of exogenous genetic material which can be added islimited by the amount which will be absorbed without being physicallydisruptive. Generally, the volume of exogenous genetic material insertedwill not exceed about 10 picoliters. The physical effects of additionmust not be so great as to physically destroy the viability of thezygote. The biological limit of the number and variety of DNA will varydepending upon the particular zygote and functions of the exogenousgenetic material and will be readily apparent to one skilled in the art,because the genetic material, including the exogenous genetic material,of the resulting zygote must be biologically capable of initiating andmaintaining the differentiation and development of the zygote into afunctional organism.

The number of copies of a transgene (e.g., the exogenous geneticmaterial or targeting vector constructs) which are added to the zygoteis dependent upon the total amount of exogenous genetic material addedand will be the amount which enables the genetic transformation tooccur. Theoretically only one copy is required; however, generally,numerous copies are utilized, for example, 1,000-20,000 copies of atargeting vector construct, in order to insure that one copy isfunctional.

Reimplantation is accomplished using standard methods. Usually, thesurrogate host is anesthetized, and the embryos are inserted into theoviduct. The number of embryos implanted into a particular host willvary by species, but will usually be comparable to the number of offspring the species naturally produces.

Transgenic offspring of the surrogate host may be screened for thepresence and/or expression of an exogenous polynucleotide (e.g., that ofa targeting vector) by any suitable method as described herein.Alternative or additional methods include biochemical assays such asenzyme and/or immunological assays, histological stains for particularmarker or enzyme activities, flow cytometric analysis, and the like.

Progeny of the transgenic animals may be obtained by mating thetransgenic animal with a suitable partner, or by in vitro fertilizationof eggs and/or sperm obtained from the transgenic animal. Where matingwith a partner is to be performed, the partner may or may not betransgenic and/or a knockout; where it is transgenic, it may contain thesame or a different knockout, or both. Alternatively, the partner may bea parental line. Where in vitro fertilization is used, the fertilizedembryo may be implanted into a surrogate host or incubated in vitro, orboth. Using either method, the progeny may be evaluated using methodsdescribed above, or other appropriate methods.

Retroviral infection can also be used to introduce a targeting vectorinto a non-human animal. The developing non-human embryo can be culturedin vitro to the blastocyst stage. During this time, the blastomeres canbe targets for retroviral infection (Jaenich, R. (1976) PNAS73:1260-1264). Efficient infection of the blastomeres is obtained byenzymatic treatment to remove the zona pellucida (Manipulating the MouseEmbryo, Hogan eds. (Cold Spring Harbor Laboratory Press, Cold SpringHarbor, 1986). The viral vector system used to introduce the targetingvector is typically a replication-defective retrovirus carrying theexogenous nucleic acid (Jahner et al. (1985) PNAS 82:6927-6931; Van derPutten et al. (1985) PNAS 82:6148-6152). Transfection is easily andefficiently obtained by culturing the blastomeres on a monolayer ofvirus-producing cells (Van der Putten, supra; Stewart et al. (1987) EMBOJ. 6:383-388). Alternatively, infection can be performed at a laterstage. Virus or virus-producing cells can be injected into theblastocoele (Jahner et al. (1982) Nature 298:623-628). Most of thefounders will be mosaic for the targeting vector (e.g., the exogenousnucleic acids) since incorporation occurs only in a subset of the cellswhich formed the transgenic non-human animal. Further, the founder maycontain various retroviral insertions of the transgene at differentpositions in the genome which generally will segregate in the offspring.In addition, it is also possible to introduce transgenes into the germline by intrauterine retroviral infection of the midgestation embryo(Jahner et al. (1982) supra).

The cre-lox system, an approach based on the ability of transgenic mice,carrying the bacteriophage Cre gene, to promote recombination between,for example, 34 by repeats termed loxP sites, allows ablation of a givengene in a tissue specific and a developmentally regulated manner (Orbanet al. (1992) PNAS 89:6861-6865). LoxP sites can be placed flanking anexon of any given gene. Thus, transgenic mice carrying the Cre geneunder the control of a selected promoter can be crossed with transgenicmice carrying a transgene flanked by loxP sites to generate doublytransgenic mice. The pioneering work in developing this system wascarried out by Orban et al. (1992) PNAS 89:6861-6865. In one embodiment,the invention uses this technology to target specific tissues in mice(e.g., expressing CMAH), in a developmentally regulated fashion in orderto produce a mouse lacking Neu5Gc.

Gene targeting producing gene knock-outs allows one to assess in vivofunction of a gene which has been altered and used to replace a normalcopy and to generate knockout animals with utility as food. Themodifications include insertion of mutant stop codons, the deletion ofDNA sequences, or the inclusion of recombination elements (lox p sites)recognized by enzymes such as Cre recombinase. The Cre-lox system asused in one embodiment of the present invention allows for the ablationof a given gene or the ablation of a certain portion of the genesequence. The Cre-lox system was used to generate CMAH knockout miceexhibiting reduced Neu5GC.

In another aspect, the invention relates to the use of an CMAH knockoutanimal, in particular an animal used for food stuff to generate food,food products, pharmaceuticals, and biologics for use.

In a further embodiment, the invention relates to cells and tissues thatcarry mutations in CMAH. The cells can be primary cells or establishedcell lines obtained from the transgenic animals of the inventionaccording to routine methods, i.e. by isolating and disintegratingtissue.

Such cells and tissues derived from the animals of the invention, inwhich the activity of CMAH has been reduced or abolished, are useful inin vitro methods relating to the study of sialic acid moieties, bindingand diseases and disorders related thereto.

Cells and cell lines derived from the knockout animals are furtheruseful in screening systems. The invention demonstrates that knockout ofCMAH results specific decreases of Neu5Gc. No obvious morphologicaldefects were noted in CMAH knockout mice.

EXAMPLES Example 1

Neu5Gc and Neu5Ac were respectively purchased from Inalco Spa (Milano,Italy) and Pfanstiehl Laboratories, Inc (Waukegan, Ill.).1,2-diamino-4,5-methylene dioxybenzene (DMB), Chlorpromazine, Genistein,Nystatin, Amiloride, and Saponin were from Sigma-Aldrich (St Louis,Mo.). Premium Human Serum type AB was from Irvine Scientific (Santa Ana,Calif.). Neu5Ac aldolase came from ICN (Costa Mesa, Calif.). All thereagents used were HPLC grade.

Cell lines—Caco-2 cells (human epithelial cells isolated from a primarycolon carcinoma), normal human skin fibroblasts (CCD-919-SK) and ChineseHamster Ovary (CHO-K1) cells were purchased from ATCC (Manassas, Va.).Mutant human skin fibroblasts (GM05520, 0M08496 and GM01718) wereobtained from the Coriell Institute for Medical Research (Camden, N.J.).Chimpanzee Fred and human LB EBV-transformed lymphoblasts were a giftfrom Dr. Peter Parham, Stanford University.

Cell culture—Caco-2 cells were propagated in alpha-MEM containingGlutamax™ and a mixture of ribonucleosides and deoxyribonucleosides(Gibco/Invitrogen) supplemented with 20% FCS. All the fibroblast celllines and CHO-K1 cells were cultured in the same media supplementedrespectively with 15% non-heat inactivated FCS or 10% heat-inactivatedFCS. Chimpanzee Fred and human LB EBV-transformed lymphoblasts werecultured in RPMI-1640 (Gibco/Invitrogen) supplemented with 10% heatinactivated FCS or 15% human serum. All the cultures were maintained at37° C., 5% CO₂ atmosphere. In order to deplete any remaining Neu5Gc fromFCS, the cells were split and cultured, prior to Neu5Gc feedingexperiments, for at least 4 days in alpha-MEM supplemented with anadequate percentage of heat-inactivated premium human serum instead ofFCS. The cells were then maintained under the same conditions during thewhole feeding experiment. The human serum was heat inactivated at 56° C.for 30 min before use.

Preparation of ManNGc from Neu5Gc—ManNGc was prepared by incubating 73μmoles of Neu5Gc with 624 U Lactate dehydrogenase, 30 μmoles NADH and 10U Neu5Ac Aldolase, EC 4.1.3.3, in 15 ml of 100 mM Potassium phosphatebuffer, pH 7.2. The incubation was carried out at 37° C. for 16 h. TheManNGc was separated from any unreacted Neu5Gc by passing the productserially over AG50WX-2 and AG1X-8 (Bio-Rad, Richmond, Calif.)ion-exchange resins as previously described. The run-through and 5column-volumes of water washes were collected and concentrated byfreeze-drying. The reaction yield (91-98%) was followed by thedisappearance of Neu5Gc, using DMB derivatization of the reactionmixture and analysis by HPLC (see protocol described below).

Preparation and purification of Sia from bovine submaxillary mucin—Amixture of standard Sias were prepared from bovine submaxillary mucin.Total mucins were extracted from frozen submaxillary glands aspreviously described. Sias then were released with mild acid, collectedby dialysis (1000 daltons molecular-weight-cut-off) and purified on ionexchange columns, under conditions determined to minimize loss ofO-acetylation.

Neu5Gc and ManNGc feeding experiment—Neu5Ac, Neu5Gc or ManNGc weredried, dissolved in the appropriate media supplemented withheat-inactivated human serum, sterilized using a Spin-X® (Coming Inc.,Corning, N.Y.) and then added to the cells. The pH of the mediacontaining Sia, was adjusted to neutrality using sterilized 1 M NaOHbefore starting the feeding experiment. Cells were cultured in thepresence of up to 3 mM free Sia or ManNGc for 1 or 3 days at 37° C. Atthe end of the feeding, cells were washed with cold PBS, harvestedeither by scraping or with 2 mM EDTA for fibroblasts, and washed againwith cold PBS prior to fractionation.

Fractionation of the labelled cells—Washed cell pellets were sonicatedinto 500 μL of 20 mM sodium phosphate buffer or 20 mM Tris-HC1, pH 7.5,using 4×15 second pulses of a sonicator cell disrupter, model SonicDismembrator (Fisher Scientific) at a probe setting of 3. The sonicatewas centrifuged at 75×g for 15 min, and the pellet obtained consistedprimarily of nuclei and unbroken cells. The pellet contained <5% of theincorporated sialic acid, as determined using a radioactive tracer (datanot shown). The supernatant was therefore considered as the “TotalHomogenate” fraction. A portion (20%) of the “Total Homogenate” fractionwas taken for protein quantification and Sia analysis by DMBderivatization and HPLC analysis (See protocol below). The remainder wascentrifuged at 100,000×g for 1 h. The resulting pellet, called the“membrane” fraction, was then resuspended by sonication (15 sec) in 200μL of sodium acetate buffer, pH 5.5. The 100,000×g supernatant, calledthe “soluble” fraction was adjusted to 90% ethanol using absoluteice-cold ethanol, and placed overnight at −20° C. The flocculantprecipitate, which represents the “soluble protein” fraction, was washed3 times with 90% of ice-cold ethanol and then, resuspended in 200 μL ofwater. The supernatant fluid representing the cytosolic low molecularweight (LMW) fraction was dried and brought up in 100 μL with waterprior to Sia analysis. All the Sias in these fractions were releasedwith mild acid hydrolysis if necessary and then analysed by HPLC, afterDMB derivatization. Protein quantification was performed on the totalhomogenate, membrane and soluble protein fractions by using the BCAprotein assay kit from Pierce (Rockford, Ill.). In some experiments, theresulting data obtained for the Sia bound to the membrane fraction andto the soluble proteins were pooled and presented here as a HighMolecular Weight (HMW) fraction.

Sialic acid release, DMB derivatization and HPLC analysis—The bound Siasfrom the total homogenate, membrane and soluble protein fractions werereleased using 2M acetic acid hydrolysis, 3 h at 80° C. The releasedSias, or free Sias contained in the soluble LMW fraction were passedthrough a Microcon® YM-10 (Millipore, Bedford, Mass.) prior to DMBderivatization, which was done according to Hara et al., 1989. DMB-Siaderivatives from the different fractions were then analysed by HPLCusing a C18 column (Microsorb MV-TM 100 A, Varian). Isocratic elutionwas achieved using 7% Methanol, 8% Acetonitrile in water during 50 minat 0.9 ml/min flow. The eluant was monitored by fluorescence asdescribed.

Quantification of Sias—For all HPLC chromatograms, the quantification ofSias was done by comparison with known quantities of DMB derivatizedNeu5Gc and Neu5Ac used as standards and then reported in terms of pmolesof Sia. For total homogenate, membrane and cytosolic protein fractions,this number was expressed per mg of protein. Due to minorsample-to-sample variations in amounts and recoveries, the data in theFigures is presented as percent of Neu5Gc over total Sias, rather thanas absolute amounts.

MS and MS/MS analysis of DMB derivatives—In some experiments, the natureof the DMB derivatives of Sias was confirmed by mass spectrometry on aFinnigan MAT HPLC with online mass spectrometry system using a modelLCQ-Mass Spectrometer System A. A Varian C18 column was used and elutedin the isocratic mode with 8% acetonitrile, 7% methanol, 0.1% formicacid in water at 0.9 ml/min over 50 min. The eluant was simultaneouslymonitored by UV absorbance at 373 nm and by Electrospray Ionization massspectrometry. The ESI settings used were capillary temperature of 210°C., capillary voltage at 31 V and the lens offset voltage at 0 V.Spectra were acquired by scanning from m/z 150-2000 in the positive ionmode. In some instances, MS/MS was acquired by selecting the parent massand using a 20% normalized collision energy. Data analysis was performedusing the XCALIBUR data analysis program from the instrumentmanufacturer.

Endocytosis Inhibition experiments—Caco-2 or normal fibroblast cellswere split and cultured in alpha-MEM media supplemented respectivelywith 20% or 15% human serum for 4 days before starting the endocytosisdrug inhibition experiments, in order to deplete any Neu5Gc derived fromFCS. Cells were then pre-treated for 2 h with the specific inhibitors,under the same culture conditions. Fresh media containing the sameamount of inhibitor and 3 mM of Neu5Gc was then added to the cells,which were incubated for 16 h or 3 days and finally harvested andfractionated as described above. Based on prior literature,Chlorpromazine, Genistein, Nystatin and Amiloride were used at finalconcentrations of 6 μg/mL, 200 μM, 25 μg/mL and 3 mM, respectively.

Western blot analysis—Membrane proteins extracted from Neu5Gc-fed,ManNGc-fed or non-fed human wild-type (WT) and mutant fibroblasts wereseparated by SDS-PAGE electrophoresis using an 8% polyacrylamide gel.The separated proteins were transferred onto nitrocellulose membrane,which was blocked overnight with Tris Buffer Saline containing 0.1% ofTween-20 (TBS-T). Immunodetection was then performed by using ourrecently described anti-Neu5 Gc antibody (1:10,000 in TBS-T, 3 h, roomtemperature (RT)). Binding of the anti-Neu5Gc antibody was detectedusing a secondary HRP-conjugated donkey anti-chicken IgY antibodydiluted at 1:30,000 in TBS-T for 45 min at RT (Jackson ImmunoResearchLaboratories, West Grove, Pa.). Final development of the blots wasperformed by using Supersignal West Pico ECL reagent (Pierce, Rockford,Ill.) and X-OMAT Kodak films.

Flow Cytometry—Human WT and mutant fibroblasts, grown in media with 10%FCS +20% horse serum for 3 days, were lightly trypsinized (0.04%trypsin, 0.53 mM EDTA for 5 min) to release cells from the flasks. Thecells were washed with PBS and then fixed overnight with 1%paraformaldehyde in PBS. Fixed cells were permeabilized or not with 0.1%saponin in PBS at RT for 20 mM. Chicken anti-Neu5Gc antibody was addedto cells at a 1:200 dilution in PBS and incubated at RT for 30 min.Cells were then washed with PBS and resuspended in FITC-conjugated goatanti-chicken IgY (1 μg/100 μl) (Southern Biotechnology Associates,Birmingham, Ala.) and allowed to incubate for 30 min at RT. Labelledcells were washed with PBS and resuspended in 500 μl PBS for analysis ofFITC fluorescence on a FACS Calibur (BD Biosciences, San Jose, Calif.).

Fluorescence Microscopy—Human WT and mutant fibroblasts were grown onpoly-D-lysine-coated glass Chamber Slides (Nalge Nunc International,Naperville, Ill.) with media containing 10% FCS+20% horse serum for 4days. Cells were fixed onto slides using 1% paraformaldehyde in PBS for30 mM at RT before permeabilizing with 0.1% saponin for 20 min at RT.Chicken anti-Neu5Gc antibody was then added at 1:50 dilution in PBSalong with 1 μg of mouse anti-LAMP-1 (clone H4A3, BD Pharmingen, SanDiego, Calif.) and incubated at RT for 1 h. Bound antibodies were thendetected with FITC-goat anti-chicken IgY and Cascade Blue-goatanti-mouse IgG (each at 1 μg/100 μl) at RT for 1 h. Cells were washedwith PBS and covered with Gel/Mount (Biomedia, Foster City, Calif.)before fluorescence imaging with a Zeiss Microscope at 400×magnification with emission filters at 400 and 520 nm for Cascade Blueand FITC, respectively.

Free Neu5Gc can be taken up by human epithelial cells from an exogenoussource and incorporated into different subcellular fractions. Evidencewas presented suggesting that the small amounts of Neu5Gc found in somehuman tissues originated from dietary sources and showed that humanCaco-2 cells (human epithelial cells from a primary colon carcinoma) inculture could metabolically incorporate free Neu5Gc, as determined by aWestern blot of a total homogenate, using and anti-Neu5Gc antibody.Increasing incorporation of Neu5Gc was found in the total homogenatefraction of the cells over time, with the highest level reached afterincubation with 3 mM Neu5Gc for 3 days. Moreover, Western blotting withan anti Neu5Gc antibody demonstrated metabolic incorporation of Neu5Gcinto glycoproteins of these cells. The partitioning of the exogenousNeu5Gc into different subcellular fractions of these cells has now beenstudied. Prior to feeding, Caco-2 cells were split and cultured in humanserum instead of FCS, in order to eliminate traces of Neu5Gc in thecells. Culture was continued for 3 days in the presence of 3 mM Neu5Gc,using 3 mM ManNGc and NeuSAc as positive controls. Indeed, it hasalready been shown that Neu5Ac and ManNGc can be incorporated into cellsand that ManNGc or its peracetylated form can be metabolized intoNeu5Gc. After the 3 day feeding, the cells were harvested and the Neu5Gccontent of the different subcellular fractions analyzed by DMBderivatization, HPLC, MS and MS/MS analysis. As shown in FIG. 1A, theDMB-HPLC profiles of Sias released from the membranes of Caco-2 cellsfed with 3 mM ManNGc or Neu5Gc presented two peaks which correspond toNeu5Gc and Neu5Ac, by comparison with the retention times of standards.Cells that were not fed or fed with Neu5Ac had only one peakcorresponding to Neu5Ac. These results were confirmed by LC-MS and MS/MSanalysis. DMB-Neu5Gc and DMB-Neu5Ac adducts gave signals at m/z 442, 424and 426, 408 respectively, representing molecular ions ofDMB-derivatized Neu5Gc and Neu5Ac and their dehydrated forms. LC-MS andMS/MS data obtained on DMB-derivatized Sias released from the membranesof Caco-2 cells non-fed or fed with Neu5Ac gave only a single ion at m/z426 which can be broken down to 408 by MS/MS, confirming the presence ofNeu5Ac and the absence of Neu5Gc. The same analysis on Neu5Gc or ManNGcfed Caco-2 cell membrane Sias gave ions at m/z 442 and 426, which arerespectively dehydrated to 424 and 408 in MS/MS analysis. These analysesconfirmed the presence of Neu5 Gc associated specifically within theglycoconjugates of the membranes of Caco-2 cells fed with ManNGc orNeu5Gc. All other sub-cellular fractions were also studied using thesame DMB-HPLC approach. Due to sample-to-sample variations in amountsand recoveries, we present the data in this and subsequent figures aspercent of Neu5Gc over total Sias, rather than as absolute amounts. FIG.1B summarizes the results showing that the total homogenate (TH), HighMolecular Weight fraction (HMW is the combination of membrane andsoluble protein fractions) and cytosolic Low Molecular Weight (LMW)fraction contain 58, 46 and 70% Neu5Gc respectively. In the experimentpresented here, the % of Neu5Gc in the membrane fraction of cells fedwith Neu5Gc was lower compared to the one obtained for cells fed withManNGc. This was not always the case, as was observed in other feedingexperiments that free Neu5Gc can be as efficient as ManNGc and sometimeseven better. The relative percentages obtained for the other fractions(total homogenate, LMW and soluble protein) were similar in severalrepeated ManNGc and Neu5Gc feeding experiments.

The uptake mechanism of free Neu5Gc is not specific for human epithelialcells. The above experiments showed that free Neu5Gc can be taken up byhuman epithelial carcinoma cells from the media, and incorporated intodifferent subcellular fractions such as membrane-bound glycoconjugates,soluble proteins, and low molecular weight compounds present in thecytosol. To see if this is a specialized mechanism in human carcinomacells, similar Neu5Gc feeding experiments were done on other human celltypes such as normal skin fibroblasts and neuroblastomas. It was foundthat fibroblast cells can also take up free Neu5Gc from the media,albeit in a less efficient manner. As presented in FIG. 2A, 28%, 39% and41% Neu5Gc are present in the TH, HMW and cytosolic LMW fractions of thehuman normal fibroblasts after Neu5Gc feeding. Lower levels (4%, 19%,12% Neu5Gc) were already present in the same fractions when fibroblastcells were not incubated in presence of 3 mM Neu5Gc. This Neu5Gc isassumed to be derived from Neu5Gc on FCS glycoproteins used for cellculture, prior to feeding experiments (see below). Human neuroblastomacells could also incorporate Neu5Gc with an efficiency comparable to theCaco-2 cells (FIG. 2B). These data indicate that the uptake mechanism ofNeu5Gc can also occur in other human cell types, with varyingefficiencies.

The uptake mechanism of free Neu5Gc is not specific for human cells.

To ask if the uptake mechanism of free Neu5Gc is specific for humancells, Neu5Gc feeding of human and chimpanzee lymphoblasts was compared.Humans are evolutionarily most closely related to the chimpanzee, whoseproteins are ˜99% identical to those of humans. Of course, great apessuch as chimpanzees are able to express Neu5Gc in large amounts becausethey have an active form of the CMP-Neu5Ac hydroxylase. Prior to thefeeding experiment, both cell types were split and cultured in humanserum instead of FCS for a couple of weeks. As expected, the Neu5Gccontent of chimpanzee lymphoblasts could not be eliminated completelybecause of the endogenous production of Neu5Gc. After a 3 day feeding of3 mM Neu5Gc or Neu5Ac, the cells were harvested, fractionated and, theNeu5Gc content of the different subcellular fractions were analysed. Asshown in the FIG. 3A, the human cells fed with 3 mM Neu5Gc contained 67%Neu5Gc in the TH, 51% in the HMW fraction and 80% in the LMW cytosoliccompounds. In contrast, the same cells had almost no detectable Neu5Gcwhen they were non-fed or fed with Neu5Ac (FIG. 3A). With chimpanzeecells, we measured baseline levels at 57% Neu5Gc in the TH, 66% in theHMW fraction and 70% in the LMW fraction of non-fed cells (FIG. 3B),representing the endogenous production of Neu5Gc by these cells, Whenthe chimpanzee lymphoblasts were fed with 3 mM Neu5Ac, the percentagesof Neu5Gc present in the different fractions changed only minimally(FIG. 3B), presumably because of biosynthetic transformation of Neu5Acto Neu5Gc occurring at the sugar nucleotide level. When the chimpanzeelymphoblasts were fed with 3 mM Neu5Gc, we observed an increase abovethe baseline levels, to 70% for the TH, 72% for the HMW fraction and 84%for the LMW fraction (FIG. 3B). Similar experiments have been done withCHO-K1 cells and with epithelial cells isolated from a spontaneous tumorfrom a CMAH gene knock out mouse. Since all these experiments gavesimilar results, we can conclude that the uptake mechanism of Neu5Gc isnot specific for human cells but can also take place in other mammaliancells. However, since non-human cells often have large endogenousamounts of Neu5Gc, the consequences are more dramatic in human cells.

Free Nen5Ac and Neu5Gc are taken up and incorporated by the samepathways.

From these data and from other recent reports in the literature, it isconcluded that Neu5Ac and Neu5Gc can be taken up by many kinds of cellsfrom an exogenous source and incorporated into endogenousglycoconjugates. Several prior studies indicate that Neu5Gc and Neu5Acare used interchangeably by essentially all of the steps leading totheir final incorporation into glycoconjugates. Higa and Paulson showedthat CMP-Sia synthetases from calf brain and from bovine and equinesubmaxillary glands both converted Neu5Ac and Neu5Gc to their CMPderivatives efficiently. They also studied six mammaliansialyltransferases purified from porcine, rat, and bovine tissues andconcluded that CMP-NeuAc and CMP NeuAc were equally good donorsubstrates for all the enzymes. Schauer and colleagues showed that thefrog liver CMP-Sia synthetases had very similar Km values for Neu5Ac andNeu5Gc. In our own prior work we concluded that CMP-Neu5Gc andCMP-Neu5Ac could be taken up by Golgi vesicles and incorporated into theendogenous glycoproteins at an approximately equal rate. Similarobservations were made by Lepers etc al. in rat and mouse liver Golgi.Thus, by doing competition experiments in Caco-2 and human normalfibroblast cells we could determine whether Neu5Gc and Neu5Ac are takenup and incorporated via the same pathways. Both cell lines gave similarresults, and only the results for Caco-2 cells are presented in FIG. 4.Feeding was done for 3 days with 3 mM Neu5Gc in the absence or presence(3 mM or 15 mM) of Neu5Ac in the media. The baseline incorporation of56% Neu5Gc in the TH was reduced to 48% in the presence of 3 mM Neu5Acand further decreased to 35% in the presence of added 15 mM Neu5Ac (FIG.4A). The percentage of Neu5Gc was even more affected in themembrane-bound fraction, reducing from 41% to 29.9% with 3 mM Neu5Ac,and almost to zero in the presence of 15 mM Neu5Ac (FIG. 4B). Since a5-fold excess of Neu5Ac was enough to abolish the incorporation ofNeu5Gc into the membrane fraction of the cells, it is concluded thatboth molecules likely use the same pathways to enter into human cellsand become available for metabolic incorporation. It is of coursepossible that there are minor differences in utilization of Neu5Gc andNeu5Ac by various enzymes and transporters in the pathways.

Free Neu5Gc Enters into Cells Via Pathways of Endocytosis.

Negatively charged hydrophilic molecules like sialic acids usually donot cross membranes. To understand how free Neu5Gc enters into cells, weexplored the hypothesis that it does so via endocytic pathways. Thus,Neu5Gc feeding experiments on Caco-2 cells were done in the presence ofdrugs that are known to inhibit various endocytic pathways common tomost cell types. Based on literature, it was decided to useChlorpromazine for blocking the clathrin dependent pathway and Nystatinand Genistein for the clathrin independent pathways (with an additionalspecific action of Nystatin on caveolar uptake). Amiloride was used asan inhibitor of fluid phase pinocytosis. All these drugs were used atconcentrations described in the Experimental Procedures and based onprior literature. As before, the Caco-2 cells were incubated in anappropriate media containing human serum instead of FCS and pre-treatedwith the drug for 2 hours, followed by the addition of 3 mM of Neu5Gcfor 16 h or 3 days. As shown in FIG. 5A, incorporation of Neu5Gc in theTH fraction, in the presence of Chlorpromazine and Nystatin (−65% inboth cases) was about the same as for the non-treated Caco-2 cells. Incontrast, Neu5Gc incorporation into cells was decreased in the presenceof Genistein (51%) and much further by Amiloride (35.4% Neu5Gc).Analysis of incorporation into membrane-bound glycoconjugates .gavesimilar results: while there was no obvious difference in the Neu5Gcincorporation for cells cultured without (45%) or with Chlorpromazine(50%) or with Nystatin (44%), Genistein and Amiloride caused markedreduction of incorporation to 34% and 10% respectively. These resultsindicate that exogenous free Neu5Gc enters cells viaclathrin-independent endocytic pathways with a major contribution fromfluid phase pinocytosis.

The Lysosomal Sialic Acid Transporter is Required for Export of FreeNeuGc from the Lysosome to the Cytosol.

Free Neu5Gc molecules entering the cell via endocytic pathways wouldstill be restricted from passively diffusing out of endosomes into thecytosol. We hypothesized that they would eventually reach the lysosome,where they would have the opportunity to utilize the previously knownlysosomal sialic acid transporter (58-60) to reach the cytosol. To testthis hypothesis, fibroblasts from a patient (GM05520) with a severeinfantile form (ISSD) of sialic acid storage disease, a disease that iscaused by a genetic defect in this transporter, were used. As shown inFIG. 6, the precent of Neu5Gc incorporation into membrane-boundglyconconjugates was reduced from 37% in normal wild-type (WT)fibroblasts to 5% in these mutant cells. As a control, the metabolicconversion of ManNGc into Neu5Gc in these cells was also studied, whichpresumably occurs following passive diffusion through the plasmamembrane, and does not require the lysosomal sialic acid transporter. Aspredicted, it was found that there was essentially no difference inbetween normal (19% Neu5Gc) versus mutant fibroblasts (18% Neu5Gc)following feeding with 3 mM ManNGc. Another similar mutant humanfibroblast cell line (GM08496) was studied, with a partial inhibition offunction of the lysosomal sialic acid transporter. This cell line wasisolated from a patient suffering from Salla disease, a milder adultform of sialic acid storage disease. Neu5Gc feeding of these cellsresulted in 16% Neu5Gc in membrane-bound glycoconjugates in comparisonto the 37% seen in normal WT fibroblasts. Again, feeding with ManNGcgave no obvious change from the control (17% Neu5Gc). To further confirmthat there was a difference in incorporation into glycoproteins, aWestern blot analysis was carried out of proteins extracted from themembranes of wild-type and GM05520 mutant human fibroblasts using ananti-Neu5Gc antibody, with or without prior Neu5Gc or ManNGc feeding. Asshown in FIG. 6B, the mutant fibroblasts could not incorporate Neu5Gcinto glycoproteins, but could in fact convert it from ManNGc. Takentogether, the data confirm the hypothesis that the lysosomal sialic acidtransporter plays a crucial role in delivering free sialic acids, thatenter into cells via endocytosis to the cytosol, for activation andincorporation into glycoconjugates.

Both the lysosomal sialidase and the lysosomal sialic acid transporterare required for incorporation of glycoprotein-bound NeuGc into Humancells.

Several studies (including this one) have shown that when human cellsare transferred from conventional media containing FCS into serum-freemedia or human serum, the small amounts of endogenous Neu5Gc in thesecells gradually disappear. It has always been assumed that this isbecause FCS contains many glycoproteins with attached Neu5Gc. However,the pathway by which these glycosidically-bound Neu5Gc molecules enterthe cell and eventually become incorporated into endogenousglycoproteins has never been defined. This question is also of directrelevance to human gut epithelial cells, which would be exposed toglycoprotein-bound Neu5Gc of dietary origin (red meat, milk products forexample). Based on the above findings, it is reasonable to hypothesizethat the Neu5Gc-carrying serum glycoproteins enter the cell via fluidphase pinocytosis, eventually reaching the lysosome, where they areexposed to the lysosomal sialidase. The resulting free Neu5Gc in thelysosome would then have the opportunity to use the lysosomal sialicacid transporter to reach the cytosol in order to be salvaged andeventually converted to CMP-Neu5Gc.

To test this hypothesis, the GM05520 mutant human fibroblasts, which arecompletely deficient in the lysosomal sialic acid transporter, as wellas GM01718 mutant human fibroblasts, which have less than 1% lysosomalsialidase activity compared to normal fibroblasts, were used. For thesestudies, it was important to differentiate between cell surface andinternal Neu5Gc. Thus, instead of subcellular fractionation, the methodof flow cytometry was utilized, using our previously described affinitypurified Neu5Gc-specific chicken antibody. As shown in FIG. 7A, after 3days of feeding with 10% FCS+20% horse serum (both rich sources ofglycoprotein-bound Neu5Gc), the total surface expression of Neu5Gc wassignificantly lower in both mutant fibroblasts compared to WTfibroblasts. Permeabilization of cells revealed similar levels of totalNeu5Gc glycoconjugates (FIG. 7A), but the majority in the two mutantswas internal (FIG. 7B). To confirm trapping of Neu5Gc glycoconjugates inlysosomes, fluorescence microscopy analysis was performed ofpermeabilized fibroblasts, co-labelling cells with a known marker forlysosomes, LAMP-1. An even distribution of Neu5Gc staining on WT normalfibroblasts with little co-localization with lysosomes (FIG. 7C) wasfound. On the other hand, both the lysosomal sialidase and thetransporter mutants demonstrated significant accumulation of Neu5Gcglycoconjugates in the lysosomes (FIG. 7C).

The results with the sialidase-deficient fibroblasts confirm ourhypothesis that this enzyme must act to release free Neu5Gc fromglycoproteins and to make it available for metabolic incorporation. Theaccumulation of Neu5Gc glycoconjugates in the lysosomal transportermutant was unexpected. A likely explanation is that accumulation of freeSia at a high concentration in the lysosomes inhibits the action of thelysosomal sialidase, resulting in accumulation of glycosidically-boundNeu5Gc. The residual levels of Neu5Gc detected on the surface of bothmutant cells might be explained by direct incorporation of gangliosidesand GPI-anchored proteins bearing Neu5GC from the serum. Taken togetherthese data indicate that bound Neu5Gc molecules that enter into humancells via pinocytosis are released by the lysosomal sialidase and arethen transported by the lysosomal sialic acid transporter to thecytosol, where they are available for activation and incorporated intoglycoconjugates (FIG. 8). Of course depending on the type ofglycoprotein involved, bound Neu5Gc could also be delivered to lysosomesvia other pathways of endocytosis, e.g., receptor-mediated endocytosisvia clathrin-coated vesicles.

It has long been assumed that free sialic acids could not be efficientlyincorporated into cells because of their negative charge and hydrophilicnature. Thus, neutral ManNAc has traditionally been used as a precursorto feed cells, for conversion into NeuSAc. The same concept has beenapplied to various unnatural mannosamine derivatives, and the additionof O-acetyl esters to the hydroxyl groups of mannosamine derivatives hasbeen used to enhance delivery across the plasma membrane. In fact, oneearly study suggested that radioactive sialic acids could beincorporated into cells, and more recent work of others has shown“efficient” uptake of a variety of kinds of sialic acids into cells.However, the kinetics of incorporation showed no evidence of saturationeven at>10 mM concentrations, suggesting that the uptake was not due toa high efficiency cell surface transporter for sialic acids. The workusing a natural sialic acid (Neu5Gc) gave similar results.

The present study resolves all these discrepancies by showing that freesialic acids from the medium can be taken up into cells vianon-clathrin-mediated mechanisms, mostly amiloride-sensitive fluid-phasepinocytosis. The content of the resulting pinocytotic vesicles andendosomes would eventually be delivered to the lysosome, where thepreviously described sialic acid transporter then delivers the moleculesinto the cytosol. We also show that the incorporation ofglycosidically-bound Neu5Gc from exogenous glycoproteins occurs bysimilar delivery to the lysosome, and release by the lysosomalsialidase, followed by export into the cytosol (FIGS. 7 and 8). Onceactivated to CMP-Neu5Gc, molecules from both sources (free andoriginally bound) would be indistinguishable from those that wereendogenously synthesized by the cells.

Most recently, it has been shown that human embryonic stem cells canincorporate Neu5Gc from medium glycoconjugates, making them targets forthe naturally occurring antibodies that circulate in most humans.Preliminary data also suggest that these antibodies could also berelated to diseases in intact humans. Thus, the mechanism by whichNeu5Gc is incorporated into human cells is of potentially greatimportance. Further studies of this process are also relevant both tothe ongoing attempts by various groups to incorporate different kinds ofunnatural sialic acids into cultured cells, and also to our efforts tounderstand how exogenous dietary Neu5Gc gain entry into normal humantissues. In this regard, it is of note that Neu5Gc accumulation appearsto be enhanced in naturally occurring tumors, and in fetal tissues. Itis suggested that this may be explained by the fact that fluid-phasemacropinocytosis is enhanced by growth factors, which are expected to bevery prominent in these two situations.

Finally, it is believed that this is the first reported example in whichan extracellular small molecule that cannot cross the plasma membrane isdelivered efficiently to the cytosol utilizing fluid pinocytosis and aspecific lysosomal transporter. The approach could thus potentially begeneralized to any small molecule that has a specific lysosomaltransporter, but not a plasma membrane transporter. For example, onecould envisage that the neutral sugars GlcNAc and GalNAc, which do nothave a high efficiency plasma membrane transporter could nevertheless bedelivered to the cytosol via the lysosomal GlcNAc/GalNAc transporter.The prediction is that adding millimolar concentrations of these sugarsinto the medium would result in significant delivery to the cytosol.

Example 2

The H1 ES cell line (WiCell Research Institute, Inc., Madison, Wis.)cells were cultured on mitotically inactivated (mitomycin C treated)mouse embryonic fibroblasts (MEF, Specialty media, Phillipsburg, N.J.)in DMEM/F12 Glutamax (Gibco, Carlsbad, Calif.), 20% “KNOCKOUT” serumreplacement (Gibco) or pooled human blood-type AB serum (Pel-Freeze,Rogers, Ariz.), 0.1 mM non-essential aminoacids (Gibco), 0.1 mMbeta-mercaptoethanol (Gibco), and 4 ng/mL BFGF-2 (R&D systems,Minneapolis, Minn.). For EB culture, H1 ES cells were grown insuspension for 7-10 days, using the same medium without FGF-2 and 10%serum. Cells were changed to a new dish every day to eliminate eventualfibroblast contamination.

HESC Transfection. H1 HESC were stably transfected to express greenfluorescent protein (GFP) by CAG-EGFP SIN lentivirus infection. The SINlentiviral vector expressing EGFP under control of the CAG promoter wasderived from a multiply attenuated HIV vector system, but included a U3deletion and introduction of a cPPT element. Vectors were produced bytriple transfection of 293 cells followed by ultracentrifugation andtitration as previously described³⁰. Undifferentiated cells were exposedto the virus at a titer of 0.5×10¹⁰ gtu/mL for 1 hour followed by a 2day recovery period. EGFP was detected by native fluorescence at day 3after transduction. Cells expressing EGFP were FACS sorted for uniformEGFP expression. No loss in EGFP expression was observed duringpropagation or EB. differentiation and up to 10 months aftertransduction. The EGFP positive cells derived from these colonies arethus polyclonal in origin. The GFP positive ES cells maintain a similarphenotype to the wild type cells (SSEA-4, SSEA-3 and Oct4-positive).

Oct-4 antibody (1:500) was from Santa Cruz (Santa Cruz, Calif.), theother marker antibodies (SSEA-3, TRA-1-60, alkaline phosphatase andnestin) were from Chemicon (Temecula, Calif.; dilution 1:100), and thesecondary Cy3 antibody from Sigma (San Louis, Mo.; dilution 1:250).Alkaline Phosphatase (AP) activity was measured using the Vector RedAlkaline Phosphatase substrate kit I from Vector laboratories(Burlingame, Calif.).

Human sera—Sera from several healthy human donors were obtained afterwritten consent and Institutional Review Board approval, and anonymouslynumbered before further use. Anti-Neu5Gc antibody levels in severalserum samples were determined as described elsewhere¹³. Two specificsera, corresponding to the lowest and highest extremes of the range,were selected for the experiments. Another serum with a high level ofanti-Neu5Gc antibodies was also tried with identical results to thosepresented in the figures.

Determination of Neu5Gc content—Sias from HESC, feeder layer cells, EBor culture medium were released by mild acid, derivatized with1,2-diamino-4,5-methylene dioxybenzene (DMB) and analyzed by HPLC todetermine the percentage of Neu5Gc in total Sias¹³.

Flow cytometry—Cells were harvested into 2 mM EDTA in phosphate buffer(PBS) and washed with PBS. 1×10⁵ cells were incubated with a chickenanti-Neu5Gc (1.5 μg/100 μL) and stained with a donkey anti-chicken IgYconjugated to Cy5 (Jackson, West Grove, Pa.; dilution 1:100 in PBS).Neu5Gc-specific antibody binding was partially blocked by co-incubationwith 1% chimpanzee serum, which (unlike human serum) is rich in Neu5Gc.

For human serum antibody deposition studies, HESC were harvested andexposed to individual human sera. Human IgGs deposited on the cells werestained with an anti-human IgG conjugated to Alexa 594 (MolecularProbes, Carlsbad, CA; dilution 1:100 in PBS). For C3b deposition, HESCwere exposed to human serum, then incubated with a goat anti-human C3b(Fitzgerald, Concord, Mass.; dilution 1:100 in PBS) and finally stainedwith an anti-goat IgG conjugated to Alexa 594 as above.

Cytotoxicity assays—A standard procedure for testingantibody:complement-mediated cytotoxicity after exposure to human serawas followed. HESC were harvested as described and resuspended intoGVB²⁺ buffer (Sigma) alone (control) or GBV²⁺ containing 25% humanserum. Cells were incubated for 2 h at 37° C. and gently shaken. Deadcells were stained with propidium iodide (5 μg/mL) and analyzed by FACS.For cytotoxicity assays on the plate, cells were exposed to serum-freeHESC culture medium containing 25% of the test human sera. After 30minutes at 37° C., they were harvested and stained with propidiumiodide.

Statistical analysis—Sia content from at least two experiments run induplicate was analyzed using the T test in Microsoft Excel. Data areexpressed as mean±standard deviation.

Presence of Neu5Gc on HESC Grown Under Standard Conditions

Neu5Gc on HESC was detected using an affinity-purified chickenpolyclonal monospecific anti-Neu5Gc antibody^(I3). HESC stablyexpressing EGFP were gated for EGFP-positivity to separate them fromcontaminating feeder layer fibroblasts. The antibody stained HESCgrowing in standard conditions, and binding was partially blocked byNeu5Gc-containing glycoproteins from chimp serum (FIG. 9 a). Blockingwas incomplete, likely because not all possible epitopes recognized bythe polyclonal antibody are present in chimpanzee serum¹³.

To chemically analyze the Sia content of HESC, we separated them fromthe feeder layer fibroblasts by FACS sorting using their EGFP signal.Feeder-layer-free EB derived from HESC were also examined withoutsorting. Both the membrane and cytosolic fractions from HESC and EB hada peak corresponding to Neu5Gc (FIG. 9 b), whose identity was confirmedby electrospray mass spectrometry (data not shown)¹³. HESC membranescontained 17.88±1.47 pmoles Sia/pig protein with 9.31±3.70 pmoles Sia/μgprotein in the cytosolic fraction. The percentage of total Sias presentas Neu5Gc varied from 6-10.5% in the membranes and from 2.5-9% in thecytosolic fraction. EB membranes had 16.59±13.88 pmoles Sia/μg proteinwith 9.13±0.10 pmoles Sia/ng protein in the cytosolic fraction. Thepercentage of total Sias present as Neu5Gc in EB varied from 5-17% forthe membranes and 6.5-11% for the cytosolic fraction.

Identifying Potential Sources of Neu5Gc in HESC

Since human cells are unable to synthesize Neu5Gc^(I2), the Neu5Gcdetected likely originated from elsewhere, eventually beingmetabolically incorporated by the HESC. As expected for other mammals,Neu5Gc represented 20% of total Sias in the mouse feeder layer(0.91±0.13 nmoles/million cells). However, uptake from feeder cellscannot explain all the Neu5Gc found in HESC, since removal of the layerto obtain EB did not eliminate it. Our ongoing studies have shown thathuman cells can take up Neu5Gc from the medium and metabolicallyincorporate it into membrane glycoconjugates. The “serumreplacement”-containing medium used to support HESC growth was found tocontain 35.93 nmoles Neu5Gc/mL, representing 54% of total Sias. Thecommercial “KNOCKOUT” “serum replacement”, used for preparing thismedium is the major source of Neu5Gc, since it contains 129 nmoles/mL.In contrast, medium without any additives is poor in Neu5Gc (0.008nmoles/mL).

Neu5Gc Content of HESC is Reduced by Growth in Heat-Inactivated HumanSerum with Low Anti-Neu5Gc Antibodies

Culture in heat-inactivated pooled normal human serum could markedlyreduce Neu5Gc in human colon carcinoma cells¹³, apparently due tometabolic replacement by Neu5Ac in the human serum. HESC was thereforeincubated in medium containing heat-inactivated human serum instead ofthe standard serum replacement (an approach already suggested by othersfor different reasons)^(14, 15). First, a lot of pooled human serum wasscreened and defined in which natural anti-Neu5Gc antibodies were verylow (hereafter called NHS). In case any residual antibodies were active,heat inactivation was used to eliminate complement. HESC incubated insuch NHS remained undifferentiated on the feeder layer, expressingtypical levels of markers of non-differentiation (alkaline phosphatase,Oct-4, S SEA-3, SSEA-4, TRA-1-60, and TRA-1-81; see examples in FIG. 10a), and lack of all differentiation markers tested (FIG. 10 a). Afterfeeder layer removal, these HESC were able to develop into normal EB.

Neu5Gc incorporation into HESC membranes dropped after 3 days (from ˜4pmoles/μg protein to 0.34±0.06 pmoles/μg protein) and down to 0.13±0.01moles/μg protein after one week (˜1% of total Sia as Neu5Gc, see FIG.11). The required presence of the mouse feeder layer apparentlyprevented complete elimination of Neu5Gc from HESC. After growing for 3days in human serum, some HESC were differentiated into EB either in 10%commercial serum-replacement, or in 10% NHS, without a feeder layer.After one week in serum-replacement, the amount of Neu5Gc on the EBmembranes increased (from 0.34±0.06 to 0.40±0.10 pmoles/ng protein). Incontrast, continued incubation in NHS further reduced Neu5Gc levels to0.047±0.06 pmoles/μg protein.

Natural Human Anti-Neu5Gc Antibodies Bind to HESC and Cause ComplementDeposition.

Healthy humans have variable levels of “natural” circulating anti-Neu5Gcantibodies¹³. We asked whether such antibodies could recognizeNeu5Gc-containing epitopes on HESC grown under standard conditions.Cells exposed to a high-level anti-Neu5Gc antibody-containing humanserum (Hi-GcAbHS) showed human IgG binding (FIG. 12 a shows only theEGFP+HESC). In contrast, staining of cells exposed to a low-levelanti-Neu5Gc antibody-containing human serum (Lo-GcAbHS) was almostsimilar to that of non-exposed controls. Antibody deposition was relatedto the amount of Neu5Gc on the HESC, since cells growing in NHS did notshow any IgG binding when exposed to the same Hi-GcAbHS (FIG. 12 b).

Cell surface antibody deposition can activate the classical complementpathway, eventually leading to killing or phagocytosis. We asked ifcomplement C3b deposition occurred following exposure to Hi-GcAbHS. Asbefore, we show the data gated for EGFP+HESC. When HESC were grown underthe standard conditions, 37% were positive for C3b (FIG. 13 c; compareto 0% background in FIG. 13 a). Only 22% of the cells were positiveafter exposure to Lo-GcAbHS, with actual levels on individual cellsbeing much lower (FIG. 13 b, note that the Y-axis is a log scale), WhenHESC were grown for 5 days in NHS-containing medium, C3b-positivityafter exposure to Hi-GcAbHS dropped to 13% (FIG. 13 c). These data areconsistent with deposition of anti-human IgG under the same conditions(FIG. 12) and also with the significant reduction in Neu5Gc on the HESCafter incubation in human serum-containing meditun(FIG. 11).

Such binding of antibody and complement to HESC would target them fordeath in vivo, via recognition by macrophages and NK cells. Regardless,attempts were made to directly determine antibody: complement-mediatedcytolysis on HESC in vitro. The standard single cell suspension requiredfor such analyses caused extensive cell death even under controlconditions (without serum). Exposure to Hi-GcAbHS caused increased deathabove background levels seen with NHS, from 40% to 60-70%. In contrast,the percentage of dead HESC after exposure to Lo-GcAbHS was similar tothat of the control. When the assay was performed directly on theculture dish for shorter time, more HESC remained alive. Cell death withHi-GcAbHS was higher than that of the control (14% vs. 10%), whereas thedeath rate after exposure to Lo-GcAbHS remained unchanged.

HESC and EB can incorporate the non-human Sia Neu5Gc from the murinefeeder layer and/or the medium, leading to an immune response mediatedby “natural” anti-Neu5Gc antibodies present in most humans. In effect,HESC appear like animal cells to the human immune system. Pooled,heat-inactivated human serum selected for low titers of anti-Neu5Gcantibodies could be substituted for the traditional animal serum orserum replacement, supporting the undifferentiated growth of HESC. Thisapproach markedly reduced the immune response, by reducing the Neu5Gccontent on the HESC.

Most existing HESC lines (including all those that are currentlyapproved for study under federal funding in the US) have been grown orderived with mouse feeder layer¹⁰. Standard culture conditions alsoinclude animal serum, or a serum replacement. It is shown here that thecommercial “serum replacement” is also a rich source of Neu5Gc, and bothHESC and EB are able to incorporate it. While the composition of thisserum replacement is under an international patent (WO 98/30679), theformulation includes proteins like transferrin, which are likely to befrom animal sources and therefore, would carry Neu5Gc. Human orthologsor recombinant proteins synthesized in bacteria could be used instead.

Many efforts have been recently made to eliminate these animal-derivedcomponents¹⁶. The use of a feeder-free system, such as Matrigel or othercomponents of the extracellular matrices, have been explored^(6,17,18).However, feeder-free conditions seem to facilitate in vitro evolution ofHESC, selecting for aneuploid cells¹⁹. Moreover, many of the medium andmatrix components are still from animal sources and contamination withNeu5Gc can be expected.

Human feeders of different origins have also been tried²⁰⁻²². Richardset al. first reported successful derivation and culture of some HESClines in the complete absence of non-human components, using feederlayers from human tissues with human serum and supplements, furtherdemonstrating the ability to develop teratomas, i.e., confirming themaintenance of pluripotentiality. It was also noted that human serum didnot cause any change in the undifferentiated state of the HESC. Othershave also tried similar xeno-free techniques on hematopoietic stem cellsby growing them on human stromal cells and using medium containing humanAB serum. Of course, the use of an “all-human” environment carries adifferent set of risks (unexpected contamination with novel or newlyemerging pathogens).

There are also potential implications for the incorporation of Neu5Gcwith regard to general HESC biology. Many characteristic markers of HESC(SSEA-3, SSEA-4, TRA-1-60, and TRA-1-81) are glycolipids orglycoproteins, many of which can carry Sias. SSEA-4, which is highlyexpressed even in long-term cultures of HESC, is the sialylated form ofthe globo-series glycolipid SSEA-3 (Gb5) TRA-1-60 is a sialylatedkeratan sulfate protein and the TRA-1-81 epitope became accessible onlyafter sialidase treatment. Neural lineage cells derived from EB expressthe polysialylated form of NCAM, as well as the antigen A2B5²⁶, whichcorresponds to polysialylated gangliosides. Since Sias are involved inself-recognition events, the presence of Neu5Gc instead of Neu5Ac couldlead to unexpected impairments of cell function and tissue development.

Another possible solution is growth in heat-inactivated serum from theactual patient who is going to receive the therapy. Similar alternativeshave been suggested for hematopoietic stem cells. Even if the patientserum contains anti-Neu5Gc antibodies, heat inactivation could preventcomplement activation, until such time as the pre-existing Neu5Gc in theHESC is metabolically eliminated by the Neu5Ac in the serum. An addedadvantage to this approach is that it would screen for allogeneiccytotoxic antibodies in the recipient's serum.

The examples set forth above are provided to give those of ordinaryskill in the art with a complete disclosure and description of how tomake and use the preferred embodiments of the invention, and are notintended to limit the scope of what the inventors regard as theirinvention. Modifications of the above-described modes for carrying outthe invention that are obvious to persons of skill in the art areintended to be within the scope of the following claims. Allpublications, patents, and patent applications cited in thisspecification are incorporated herein by reference as if each suchpublication, patent or patent application were specifically andindividually indicated to be incorporated herein by reference.

Attached hereto are appendicies A and B that are further meant toillustrate and not limit the invention.

1. A method for producing animal products devoid of N-glycolylneuraminicacid (Neu5Gc) for human use comprising the steps of: preparing agenetically altered non-human mammal lacking a functionalcytidine-5′-monophosphate N-acetyl neuraminic acid hydrolase (CMAH)gene; and extracting at least one animal product from the geneticallyaltered non-human animal.
 2. The method according to claim 1, whereinthe CMAH gene is disrupted by a frame-shift mutation.
 3. The methodaccording to claim 1, wherein the CMAH gene is disrupted by applicationof a cre-lox system mutation.
 4. The method according to claim 1,wherein the non-human mammal is ovine, bovine, piscine or porcine. 5.The method according to claim 1, wherein the CMAH gene is mutated in theRieske iron-sulfur center to diminish gene function.
 6. The methodaccording to claim 5, wherein the CMAH gene is mutated at the site ofthe Cys, His, or both Cys and His residues in the Rieske iron-sulfurcenter.
 7. The method according to claim 1, wherein the CMAH gene ismutated in at least one of the mononuclear iron center binding sites. 8.The method according to claim 1, wherein the CMAH gene is mutated in theCMP-Neu5Ac binding site.
 9. The method according to claim 1, wherein theCMAH gene is mutated in the cytochrome b5 binding site.
 10. The methodaccording to claim 1, wherein the non-human animal product is selectedfrom the group consisting of: serum, muscle, tissue and milk.
 11. Atransgenic non-human mammal comprising a CMAH gene lacking a functionalcopy of at least one of the gene domains associated with enzyme activityselected from the group consisting of: a Rieske Iron Sulfur center, amononuclear iron center binding site, a CMP-Neu5Ac binding site, and acytochrome b5 binding site; wherein said mammal produces animal productslacking Neu5Gc.
 12. The transgenic non-human mammal of claim 11, whereinthe non-human mammal is selected from the group consisting of ovine,bovine, piscine and porcine.
 13. A knockout non-human transgenic mammalthat lacks expression of CMAH.
 14. The knockout non-human transgenicmammal of claim 13, wherein the transgenic knockout is CMAH.
 15. Anon-human transgenic animal that carries germline mutations in a CMAHgene that disrupt CMAH activity.
 16. A cell line derived from a knockoutanimal of claim 13 non-human transgenic mammal that lacks expression ofCMAH.
 17. The cell line of claim 16 selected from the group consistingof: stem cells, epithelial cells and muscle cells.
 18. An animal productobtained from a knockout non-human transgenic CMAH^(-/-) mammal thatlacks expression of CMAH.
 19. The animal product of claim 18 selectedfrom the group consisting of tissue and serum.
 20. A human embryonicstem cell line devoid of N-glycolylneuraminic acid (Neu5Gc) comprising:a preselected line of human embryonic stem cells; and a culture mediumcontaining Neu5Gc-non-human mammalian serum from a Neu5Gc nulltransgenic non-human mammal.
 21. A tissue derived from a viable andfertile non-human animal that a) comprises a mutantcytidine-5′-monophosphate N-acetyl neuraminic acid hydrolase (CMAH) genethat encodes a mutant CMAH protein lacking enzyme activity, and b) lacksN-glycolylneuraminic acid (Neu5Gc) in one or more body fluid or tissue.22. A body fluid derived from a viable and fertile non-human animal thata) comprises a mutant cytidine-5′-monophosphate N-acetyl neuraminic acidhydrolase (CMAH) gene that encodes a mutant CMAH protein lacking enzymeactivity, and b) lacks N-glycolylneuraminic acid (Neu5Gc) in one or morebody fluid or tissue.